Sen. Marco Rubio has lost no time paying an $8,000 civil penalty to the Federal Election Commission for taking campaign donations beyond the legal limit in his 2010 Senate race. But the Florida Republican’s disputes with the agency may not be over.

The FEC has also raised questions about irregular contributions to Rubio’s leadership political action committee, the Reclaim America PAC, which has collected $826,000 since it was launched nine months ago.

In a March 9 letter to the PAC, the FEC pointed to one or more donations in excess of federal limits and to at least one prohibited corporate contribution. While a 2010 Supreme Court ruling freed corporations to spend campaign money independently, direct corporate donations to candidates and parties remain illegal.

The PAC alerted the agency in mid-April that it had made two refunds of $5,000 each to donors who exceeded the contribution limit. But FEC records don’t show any response to the request for information about corporate contributions. The FEC letter to the PAC cites donations totaling $3,250 from Van Wagner Miami, an outdoor advertising firm based in New York City.

A Rubio spokesman said by email that he would look into the matter but didn’t provide further information as of press time. He also declined to comment on the $8,000 FEC penalty, the outcome of a March settlement between Rubio and the FEC involving about $210,000 in what the commission called “prohibited, excessive, and other impermissible contributions” that were not fully refunded in time.

Such FEC penalties and inquiries are common in federal campaigns but have drawn notice in Rubio’s case because of his potential role as a running mate to presumptive GOP presidential nominee Mitt Romney. Rubio has said the vice presidency does not interest him, but he campaigned with the former Massachusetts governor in Pennsylvania last month.

Rubio has recently addressed questions about financial controversies dating to his tenure as the Speaker of the Florida House, including his use of a state Republican Party credit card to cover personal expenses. Rubio said on Fox News this week that he directly paid the credit card company for personal expenses — which reportedly included a new kitchen floor, groceries and car repairs — but he acknowledged that the arrangement “looks bad” in hindsight.

Rubio’s financial problems in the Legislature, which also included a dispute with a bank over mortgage payments and reports that he double-billed taxpayers and the state GOP for airplane tickets, didn’t hurt him during a hard-fought Senate campaign against former Republican Gov. Charlie Crist.

Rubio remains one of the most popular politicians in the state, with a 49 percent job approval rating in the latest Quinnipiac University poll, ahead of both Sen. Bill Nelson (D-Fla.) and Florida GOP Gov. Rick Scott.

Voters tend to tune out such election law disputes, said Dario Moreno, professor of political science at Florida International University, who added that Rubio’s recent FEC violations “seemed more like a case of bad bookkeeping than anything intentional.” But he noted that further allegations could prove damaging “if this becomes a consistent pattern and if other revelations come out.”

Former Florida GOP Chairman Jim Greer’s scheduled testimony in a criminal fraud trial later this summer could cause headaches for Rubio, according to political observers. Rubio has also answered questions recently about his longtime friendship with Rep. David Rivera (R-Fla.), who reportedly is under federal investigation for financial irregularities of his own.

Rubio spent $2.6 million on his Senate race in 2010, according to the nonpartisan Center for Responsive Politics, ranking him seventh in a tally of the top 10 Congressional spenders. His leadership PAC and his Senate campaign have drawn heavily on the health care, real estate and legal industries for donations, CRP data show. Koch Industries donated $32,000 to his Senate campaign in this election through its employees and its PAC and also gave the $10,000 maximum to his leadership PAC.

The leadership PAC’s letter responding to the FEC this month shows two $5,000 refunds, one to Maximo Alvarez, president of Sunshine Gasoline Distributors, and another to Mario Murgado, president and CEO of Brickell Motors, both in Miami. FEC records show two apparent corporate contributions to Rubio’s leadership PAC. In addition to the donation from Van Wagner Miami cited by the FEC, public records show a second donation from Poseidon Resources, a Connecticut-based developer of water infrastructure projects.